**Core Concept**
The patient's symptoms, such as sore throat and lymphadenopathy, in combination with a positive heterophile antibodies test, point towards a diagnosis of a specific viral infection. This condition is characterized by a sudden onset of symptoms, typically affecting the pharynx and lymphoid tissues.

**Why the Correct Answer is Right**
The positive heterophile antibodies test is a diagnostic hallmark of Infectious Mononucleosis (IM), caused by the Epstein-Barr Virus (EBV). The test detects the presence of these antibodies, which are produced in response to EBV infection. The virus targets B lymphocytes, leading to lymphadenopathy and other systemic symptoms. In IM, the heterophile antibodies are produced by activated lymphocytes and are directed against the host's own cells, leading to the characteristic symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** While a sore throat can be a symptom of many conditions, the combination of lymphadenopathy and positive heterophile antibodies test makes Infectious Mononucleosis a more likely diagnosis than a simple pharyngitis.
**Option B:** This option is incorrect as Cytomegalovirus (CMV) infection can cause a similar presentation, but it is less common in this age group and would not typically result in a positive heterophile antibodies test.
**Option C:** This option is incorrect as Streptococcal pharyngitis can cause sore throat and lymphadenopathy, but it would not result in a positive heterophile antibodies test.

**Clinical Pearl / High-Yield Fact**
Remember that a positive heterophile antibodies test is highly specific for Infectious Mononucleosis, but it can also be seen in other viral infections, such as CMV and HIV. A thorough clinical evaluation and laboratory workup are necessary to confirm the diagnosis.

**Correct Answer:** A. Infectious Mononucleosis